This invention relates to self-crosslinking cationic vinyl ester based polymer latices, a process for their preparation, and their use as binders or coatings for formaldehyde-free end use areas.
The preparation of functional cationic monomers and the use thereof in alkaline-curing polymers are described in U.S. Pat. Nos. 3,678,098, 3,694,393, and 3,702,799 (issued July 18, Sept. 26, and Nov. 14, 1972 to Sheldon N. Lewis et al.) and U.S. Pat. Nos. 3,095,390 and 3,287,305 (issued June 25, 1963 and Nov. 22, 1966 to A. Maeder).
The cationic functional monomers of the Lewis et al. patents are monoethylenically unsaturated ester monomers derived from N,N-dialkylaminoalkyl esters of (meth)acrylic acid and epihalohydrins. They have the formula ##STR1## wherein R is hydrogen or methyl; A is C.sub.2 -C.sub.6 alkylene group having at least two carbon atoms in a chain between the O and N atoms or A is a polyoxyalkylene group of the formula (CH.sub.2 CH.sub.2 O).sub.x CH.sub.2 CH.sub.2 where x is at least 1; X is iodine, bromine, or chlorine; and Y is an anion. The polymers thereof are used as wet strength agents in paper and the acrylate copolymers thereof have been suggested for use in bonding nonwoven fabrics. The polymers are prepared using typical emulsion polymerization techniques including the use of conventional initiators or redox systems and nonionic or cationic emulsifiers (surfactants).
The monomers of the Maeder patents include monoethylenically unsaturated amide monomers derived from N,N-dialkylaminoalkyl amides of (meth)acrylic acid and epichlorohydrin. These monomers have the formula ##STR2## wherein R is hydrogen or a methyl group and n is 2 or 3. Cationic dispersions of finely divided water insoluble polymers thereof (e.g. polymers with styrene) are useful for coating and impregnating textiles, for example, for rendering them water repellant. Customary polymerization techniques are used including the use of organic or inorganic peroxides or persulfates as catalysts and the use of cationic or nonionic emulsifying agents.
The major problem associated with the polymerization of vinyl esters, such as vinyl acetate, with these cationic monomers is the retardation of polymerization and the resultant poor conversion. Anionic water-soluble peroxydisulfates cannot be used as initiators since coagulation of the cationic monomer and/or copolymer results. The use of water-soluble redox initiators gives either poor conversion or latices which are not alkaline-curable to crosslinked polymers.
In order to prepare vinyl ester based polymers at high conversion, a water-soluble azo initiator may be used. One such initiator is 2,2'-azobis(2-amidino-propane) hydrochloride which may be used either with cationic or nonionic surfactants. The conversion and end use performance of the resulting polymer latices is good but the bulk viscosity of the emulsion is usually too high. Because of the water solubility of the cationic functional monomer and vinyl acetate, a water-soluble polyelectrolyte is formed in the aqueous phase during the polymerization. This water-soluble polymer is partially responsible for the high latex viscosities (independent of particle size effects). Typically, chain transfer agents such as 2-mercaptoethanol are used to reduce the bulk viscosity. The use thereof does reduce the viscosity but, because of their solubility partition coefficient, the intrinsic viscosity of the polymer is also reduced [i.e. they function to control molecular weight by chain transfer in both the polymer phase (discontinuous) or the aqueous phase (continuous)].
There is therefore a need for a water-soluble initiator/chain transfer system for use in the emulsion polymerization of vinyl acetate and the above functional cationic monomers which results in good conversion and which provides an alkaline-curing polymer latex characterized by its low bulk viscosity and the high molecular weight of the polymer therein. On the performance side, for nonwoven diaper facings, the polyester webs treated with the self-crosslinking cationic polymer latices should have good wet, dry and hot tensile strength.